Velocity modulation tube employing cascaded harmonic prebunching

ABSTRACT

A velocity modulation microwave tube is disclosed. The tube includes an input circuit, an output circuit and a penultimate resonator circuit disposed along the beam. A pair of second harmonic floating prebuncher resonators are disposed along the beam path intermediate the input circuit and the penultimate resonator. The second harmonic resonators improve the bunching of the beam by moving electrons from the interbunch region of the beam into the bunched region of the beam. A resonator tuned for a fundamental mode of resonance at the fundamental frequency of the tube is interposed between the pair of second harmonic resonators for rebunching the electron bunches of the beam downstream of the first second harmonic cavity. The second harmonic resonators with the fundamental resonator interposed therebetween substantially improve the conversion efficiency of the microwave tube.

United States Patent [72] Inventor Erliug L. Lien 2.605,444 7/1952Garbuny 315/543 [m Altos, Calif. 3,195,007 7/1965 Watson et a] 315/543 N970 Primary Examiner-Herman Karl Saalbach gg 971 AssistantExaminer-Saxfield Chatmon, Jr.

3 s [73] Assignee v A Attorneys Stanley Z. Cole Palo Alto, Calif.

ABSTRACT: A velocity modulation microwave tube is dis- VELOCITYMODULATION TUBE m I" DYING closedd Thettube include: an nputrcumdanloutpi: (:ICUII CASCADE HARMONIC PREBUNCHING an a penu ima eresona or circuit ispose a ong t e earn. 7 cm 5 D" A pair of secondharmonic floating prebuncher resonators are g disposed along the beampath intermediate the input circuit [52] US. Cl 315/543, and thepenultimate resonate The econd h rmo ic resona- 31 /5-5 5/ 5 torsimprove the bunching of the beam by moving electrons [51] Int. ClH01j25/12 from the inter-bunch region of the beam into the bunched e-[50] Field 01 Search 315/539, gion of the beam, A resonator tuned for afundamental mode 5.52 of resonance at the fundamental frequency of thetube is inter posed between the pair of second harmonic resonators forl56l References CM rebunching the electron bunches of the beamdownstream of UNITED STATE PA E the first second harmonic cavity. Thesecond harmonic 2,414,843 l/ 1947 Varian et a1. 315/552 X eso to s iththe fundamental resonator interposed 2,494,721 1/1950 Robertson et al...3 15/543 therebetween substantially improve the conversion efficiency2,579,480 12/1951 Feenberg 3 1 5/5 .43 of the microwave tube.

1 PRE BUNCHER7 J FINAL BUNCHER I *1 7 P STAGE Z STAGE /|5 A A i 1 3 f II l 8 -I 6 9 H r r r T+ {1 PATENTED JUL20 I97! OUTPUT FIG. I

+ FINAL OUNCHER PRE BUNCHER 7 PRIOR ART R OUT PUT J FINAL BUNCHE PREBUNCHER 7 FIG. 4

CASCADED HARMONIC PRE BUNCHERS INVENTOR. ERLING L. LIEN BY &Q. %Q2LATTORNEY TIME BUNOHER VOLTAGE l.O -3O '20 I0 0 NORMALTZED VELOCITYDEVIATION FOR OWEST ELECTRON IN THE BUNCH 0.75 RAD. 3L

LOO RAD.

VELOCITY MODULATION TUBE EMPLO YING CASCADED I'IARMONIC PREBUNCI-IINGDESCRIPTION OF THE PRIOR ART Heretofore, a pair of cascaded secondharmonic resonators have been disposed along the beam path intermediatethe input resonator and the penultimate resonator for improving thebunching of the beam by relocating electrons from the interbunch region.This improved bunching of the beam has substantially improved theconversion efficiency of velocity modulation microwave tubes. Thisfeature is disclosed and claimed in copending US. application Ser. No.767,774, filed Oct. 15, I968, and assigned to the same assignee as thepresent invention. While the use of the second harmonic resonatorssubstantially improves the conversion efficiency of the tube byrelocating electrons from the interbunch region, in doing so the secondharmonic resonators have a tendency to debunch the electron bunches.Therefore, it would be desirable to provide a second harmonic prebuncherof such a character that it relocates electrons from the interbunchregion without tending to debunch the electron bunches.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved velocity modulation tubeemploying harmonic prebunchers.

One feature of the present invention is the provision, in a velocitymodulation microwave tube, of a pair of cascaded harmonic prebunchers,each of such prebunchers including a fundamental mode resonator followedby a harmonic prebuncher resonator such that the fundamental resonatorof the downstream second harmonic prebuncher stage rebunches the bunchedelectrons in the beam passing into the second hannonic prebuncherresonator of the second stage of harmonic prebunching, whereby theconversion efficiency of the microwave tube is substantially increased.

Another feature of the present invention is the same as the precedingfeature wherein the pair of harmonic prebuncher resonators are tuned forthe second harmonic of the tube.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the fundamental resonators andharmonic resonators of the cascaded harmonic prebunchers are cavityresonators.

Other features and advantages of the present invention will becomeapparent upon perusal of the following specification taken in connectionwith the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram ofa multicavity klystron amplifier employing the prior art second harmonicprebuncher scheme,

FIG. 2 is a plot of buncher voltage versus time depicting the mode ofoperation of the second harmonic buncher voltage for removing electronsfrom the interbunch region,

FIG.'3 is a schematic line diagram of a velocity modulation electrontube incorporating the cascaded harmonic prebuncher stage of the presentinvention,

FIG. 4 is a plot of normalized radio frequency beam current versusnormalized velocity deviation for slowest electron in the bunches andshowing the characteristics of the prior art and of the presentinvention, and

FIG. is a plot of conversion efficiency versus normalized loadconductance for the output resonator and showing the efficiency forthree values of output interaction gap length.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, thereis shown the prior art microwave klystron tube 1 incorporating cascadedsecond harmonic prebunchers as disclosed in the aforecited copending US.application Ser. No. 767,774. Briefly, the electron tube 1 includes anelectron gun 2 for forming and projecting a beam of electrons 3 over anelongated beam path to a collector structure 4 disposed at the terminalend of the beam path. An input cavity resonator 5 is disposed at theupstream end of the electron beam 3 for velocity modulating the beamwith microwave energy supplied to the cavity 5 via input coupler 6. Anoutput cavity resonator 7 is disposed adjacent the collector 4 forinteraction with the bunched electron beam to extract amplified outputmicrowave energy from the beam. Output energy is extracted from theoutput resonator 7 via output coupling loop 8, and transmitted to asuitable utilization device, such as an antenna, not shown.

A pair of floating second harmonic cavity resonators 9 and 11 aredisposed in the prebuncher section of the electron tube. The secondharmonic cavities 9 and 11 are tuned for a fundamental mode of resonanceat a frequency which is substantially twice the center passbandfrequency of the amplifier tube 1. As used herein, floating" is definedto mean a resonator which does not have any substantial source of energyexternal to the microwave tube 1 and which is not coupled to a loadutilizing the output of the resonator; however, a circuit may be coupledto the floating resonator solely for effecting some electricalcharacteristic of the floating resonator such as its 0 or frequency.

The floating harmonic resonator 9 and the cavity resonator 5 function toproduce a bunching voltage V having a periodic sawtooth waveform asshown in Fig. 2. More specifically, the input cavity resonator 5produces a fundamental frequency component of bunching RF voltage V,.When a second harmonic voltage V is superimposed upon V the totalbunching voltage V, has a periodic sawtooth waveform. Utilizing pureelectron ballistics theory, and neglecting space charge forces, such abunching waveform produces tight bunches in the output resonator.However, such bunching produces an undesired velocity spread in theelectrons as they pass through the gap in the output resonator 7. Inaddition, in relatively high-perveance beam tubes, i.e., perveancegreater than 10', space charge forces cannot be neglected and as aresult the tight bunching which is desired may not be obtained inpractice. The floating harmonic resonator ll cancels the second harmonicvelocity modulation produced by resonator 9. By providing a finalbuncher section between the prebuncher section and the output, theelectrons are grouped by the harmonic prebuncher for a more efficientmodulation by the fundamental frequency buncher cavities 12 and 13 asused in the final buncher section. In other words, the second harmonicprebuncher cavities 9 and 11 provide a beneficial phase grouping of theelectrons at the entrance of the final buncher stage. Final bunchercavities 12 and 13 are tuned for a fundamental mode of resonance nearand above the passband of the tube and such resonators are of thefloating type. Prior art tubes of the type shown in Fig. 1 (withoutresonator 11 present) provide a calculated R.F. conversion efficiency ofapproximately percent and have yielded 70 percent conversion efficiencyin practice.

One of the problems with the prior art tube of Fig. l is that the secondharmonic prebuncher cavities 9 and I1 tend to debunch the electronbunches. This adversely affects the conversion efficiency of the tube.

Referring now to Fig. 3, there is shown a multicavity klystron amplifiertube 15 incorporating features of the present invention. The tube 15 issubstantially identical to the klystron tube 1 of Fig. l with theexception that an additional fundamental frequency resonator 16 has beendisposed along the beam path intermediate the pair of second hannonicprebuncher resonators 9 and 11. The fundamental resonator 16 is tunedfor a fundamental mode of resonance slightly above (no greater than 10percent of the center frequency of the passband) the passband of thetube and serves to rebunch the electron bunches before such bunches see"the second hannonic prebuncher voltage produced by the second stageharmonic prebuncher cavity 11. More particularly, the first prebuncherstage includes a fundamental frequency input cavity followed by thefirst second harmonic cavity 9. The second harmonic cavity 9 serves torelocate the interbunch electrons but in doing so adversely affects thedesired electron bunches tending to debunch same. However, fundamentalfrequency resonator 16 of the second prebuncher stage serves to rcbunchthe electrons prior to their entering the downstream second harmonicresonator 11. The net result of utilizing two stages of second harmonicprebunching with a fundamental resonator between the pair of secondharmonic resonators is to substantially increase the fundamentalcomponent of RP. beam current at the location of the output gap, toobtain a more uniform velocity of the electrons within the bunches andto obtain a more uniform electron distribution in the bunches. Theincreased fundamental component of the R.F. beam current versus improvedvelocity distribution within the bunches can be seen by reference toFig. 4 wherein curve 18 shows a plot of normalized fundamental frequencyRF. beam current versus normalized velocity deviation for the slowestelectron in the bunches for various gap voltages across the gap of thepenultimate resonator 12 in the tube of Fig. 3. Curve 19 shows a similarplot employing only a single stage of second harmonic prebunching, i.e.,that is without the provision of a fundamental mode resonator between apair of second harmonic prebunching resonators.

Referring now to Fig. 5, the calculated conversion efficiency for thetube 15 of Fig. 3 is shown as a function of the normalized loadconductance of the output resonator. The plots of Fig. 5 represent animprovement in conversion efficiency of approximately 5 percent ascompared to the conversion efficiency of the prior art tube asexemplified by tube 1 of Fig. 1.

In a typical example of the klystron tube 15 incorporating features ofthe present invention, the resonators are tuned as follows: the inputresonator 5 is preferably tuned essentially to the center of thepassband of the tube namely 8f/f=0 where f is the center frequency ofthe passband of the tube. The first second harmonic resonator 9 ispreferably tuned to a frequency slightly less than twice the centerbandfrequency, namely, 6fl2f=0.0045; the second prebuncher fundamentalfrequency resonator 16 is preferably tuned to a frequency slightly abovethe passband frequency, namely, 8f/f=l-0.04; the second harmonicresonator 11 is preferably tuned to the same frequency as the firstsecond harmonic resonator 9, namely 8f/2f=0.0045; the final buncherresonators l2 and 13 are preferably tuned to substantially the samefrequency slightly above the centerband frequency, namely, 8flf=l-0.022,and the output resonator 7 is preferably tuned substantially to thefrequency of the center of the passband, namely, 8f/f=0. As used herein,passband" is defined as that band of frequencies between 3 db. points inthe power output versus frequency plot for the tube.

Although the embodiment of the present invention, as depicted in Fig. 3,has employed only a single-fundamental frequency resonator 5 upstream ofthe first second harmonic resonator 9, this is not a requirement.Additional fundamental frequency resonators may be provided along thebeam path between the input resonator 5 and the first second harmonicresonator 9 for providing increased gain for the tube. In addition, itis not a requirement that the harmonic resonators 9 and 11 be tuned tothe second harmonic of the center of passband of the tube. Such harmonicresonators may be tuned to higher frequency harmonic as the third,fourth, etc. However, it is definitely preferred that the harmoniccavities be tuned to the lowest frequency harmonic higher than the first(namely the second), since with harmonics higher than the second andespecially at frequencies above S-band and at high-power levels, aninordinate proportion of the volume of the cavity is occupied by thebeam.

Moreover, it is not a requirement that the resonator circuits employedin the tube of the present invention be reentrant cavity resonators. itis contemplated that other types of resonant circuits may be employedsuch as, distributed field helical resonators (either the single helixor cross-wound helix type may be emplo ed). Moreover, the output circuitmay comprise, for examp e, a slow wave circuit or an extendedinteraction circuit formed by a plurality of coupled cavity resonators.In other words, the present invention is applicable not only to klystronamplifiers but to velocity modulation, microwave tubes in general whichemploy resonant structures disposed along the beam path for interactionwith the beam.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

lclaim:

1. in a velocity modulation microwave tube apparatus having a certainoperating passband of frequencies; means for projecting a stream ofelectrons over an elongated beam path; wave supportive means arrangedsuccessively along the beam path for successive electromagneticinteraction with the electron stream to produce output microwave energy;said wave supportive means including an input circuit, an outputcircuit, and a penultimate resonator circuit; said penultimate resonatorcircuit being tuned for a fundamental mode of resonance at a frequencyabove the passband of the tube, the frequency of said fundamental modeof resonance being less than twice the center passband frequency; saidpenultimate resonator circuit comprising at least one floating resonatorfor bunching the beam passing into said output circuit; a pair ofhannonic floating resonator means spaced apart along the beam pathintermediate said input circuit and said penultimate resonator circuit;said pair of harmonic resonator means each having a fundamental mode ofresonance approximately at a frequency corresponding to a harmonic whichis an integral multiple of the center frequency of the passband of thetube and a floating resonator means interposed along the beam pathbetween said pair of harmonic resonator means and being tuned for afundamental mode of resonance at a frequency approximately at thefundamental frequency of the passband of the tube, whereby theconversion efficiency of the tube is enhanced.

2. The apparatus of claim 1 wherein said pair of harmonic resonatormeans are tuned to frequencies approximately at the second harmonic ofthe center frequency of the passband of the tube, and wherein saidresonator interposed between said pair of harmonic resonator means istuned to a frequency above the passband of the tube.

3. The apparatus of claim 1 wherein said resonators are reentrant cavityresonators.

4. The apparatus of claim 1 wherein said pair of hannonic resonatormeans are each tuned for a fundamental mode of resonance approximatelyat the second harmonic of the center frequency of the passband of thetube.

5. The apparatus of claim 1 wherein said pair of harmonic resonatormeans are each tuned to a frequency which is twice a frequency withinthe passband of the tube but below the center frequency of the passbandof the tube 6. The apparatus according to claim 5 wherein said resonatorinterposed between said pair of harmonic resonator means is tuned to afrequency above the passband of the tube.

7. The apparatus of claim 6 wherein said penultimate resonator is tunedto a frequency within the passband of the tube but above the centerfrequency of the passband of the tube.

Disclaimer 3,594,606.-Erlin L. Lien, Los Altos, Calif. VELOCITYMODULATION EMPLOYING OASCADED HARMONIC PREBUNCH- ING. Patent dated July20, 1971. Disclaimer filed June 1, 1973, by the assignee, Vam'a nAssociates. Hereby enters this disclaimer to claim 7 of said patent.

[Oflieial Gazette November 6, 1973.]

1. In a velocity modulation microwave tube apparatus having a certainoperating passband of frequencies; means for projecting a stream ofelectrons over an elongated beam path; wave supportive means arrangedsuccessively along the beam path for successive electromagneticinteraction with the electron stream to produce output microwave energy;said wave supportive means including an input circuit, an outputcircuit, and a penultimate resonator circuit; said penultimate resonatorcircuit being tuned for a fundamental mode of resonance at a frequencyabove the passband of the tube, the frequency of said fundamental modeof resonance being less than twice the center passband frequency; saidpenultimate resonator circuit comprising at least one floating resonatorfor bunching the beam passing into said output circuit; a pair ofharmonic floating resonator means spaced apart along the beam pathintermediate said input circuit and said penultimate resonator circuit;said pair of harmonic resonator means each having a fundamental mode ofresonance approximately at a frequency corresponding to a harmonic whichis an integral multiple of the center frequency of the passband of thetube and a floating resonator means interposed along the beam pathbetween said pair of harmonic resonator means and bEing tuned for afundamental mode of resonance at a frequency approximately at thefundamental frequency of the passband of the tube, whereby theconversion efficiency of the tube is enhanced.
 2. The apparatus of claim1 wherein said pair of harmonic resonator means are tuned to frequenciesapproximately at the second harmonic of the center frequency of thepassband of the tube, and wherein said resonator interposed between saidpair of harmonic resonator means is tuned to a frequency above thepassband of the tube.
 3. The apparatus of claim 1 wherein saidresonators are reentrant cavity resonators.
 4. The apparatus of claim 1wherein said pair of harmonic resonator means are each tuned for afundamental mode of resonance approximately at the second harmonic ofthe center frequency of the passband of the tube.
 5. The apparatus ofclaim 1 wherein said pair of harmonic resonator means are each tuned toa frequency which is twice a frequency within the passband of the tubebut below the center frequency of the passband of the tube
 6. Theapparatus according to claim 5 wherein said resonator interposed betweensaid pair of harmonic resonator means is tuned to a frequency above thepassband of the tube.
 7. The apparatus of claim 6 wherein saidpenultimate resonator is tuned to a frequency within the passband of thetube but above the center frequency of the passband of the tube.